Microwave antenna having an undulating conductor with variable pitch and amplitude

ABSTRACT

The antenna array disclosed comprises an undulatory conductor which is capable of transmission and reception of energy, in which the amplitude and pitch of the undulations are progressively altered along the array to produce an array having directional characteristics, and a main lobe to side lobe ratio, which are those of a Dolph-Tchebycheff array. The undulations are circularly arcuate and blended into one another and the conductor is etched from one copper layer of a strip of low loss copper clad laminate, the other layer of which forms a ground plane.

United States Patent 721 Inventors Michael FrankCoslett App]. No. FiledPatented Assignee Priority Maldon, Essex;

Robert Frost, Wells, Somerset; Kenneth Owen Rositer, Wells, Somerset allof, England May 6, 1969 July 27, 197 1 Electric & Musical IndustriesLimited Hayes, Middlesex, England Mny 9, 1968 Great Britain MICROWAVEANTENNA HAVING AN UNDULATING CONDUCT OR WITH VARIABLE PITCH ANDAMPLITUDE 3 Claims, 3 Drawing Figs.

Int. Cl. H0111 1/36 Field olseareh 343/731 [56] Relerences Cited UNITEDSTATES PATENTS 2,759,183 7/1956 Woodward 343/806 2,990,547 6/1961McDougal... 343/7925 3,302,207 1/1967 Hoffman 343/731 3,369,246 2/1968Fisk et a1. 343/806 FOREIGN PATENTS 1,118,280 11/1961 Germany 343/806Primary Examiner-Eli Lieberman Attorney-Fleit, Gipple and JacobsonABSTRACT: The antenna array disclosed comprises an undulatory conductorwhich is capable of transmission and reception of energy, in which theamplitude and pitch of the undulations are progressively altered alongthe array to produce an array having directional characteristics, and amain lobe to side lobe ratio, which are those of a Dolph-Tchebycheffarray. The undulations are circularly arcuate and blended into oneanother and the conductor is etched from one copper layer of a strip oflow loss copper clad laminate, the other layer of which forms a groundplane.

PATENTEU JUL2 71971 sum 1 OF 3 LUAD END v i' L.

mvsnrons MICHAEL F. COSSLETT ROBERT FROST KENNETH O. ROSSITER PATENIFDM27 1971 SHEET 2 OF 3 N Q 7 N: u u 2G 92 n 35:85 2? w $252: :23 u u3:53:28 ENE: 82 22%: J51;

252%; 2265 5x535. 5 384 5:622 24 :E u 25%; Em 5 -25. ZQEI INVENTORSMICHAEL E COSSLETT ROBERT FROST KENNETH O. ROSSITER RNEYJSJ MICROWAVEANTENNA HAVING AN UNDULATING CONDUCTOR WITH VARIABLE PITCH AND AMPLITUDEThe present invention relates to microwave antenna arrays for thepurpose of transmission and reception, and especially but notexclusively to such arrays used at microwave frequencies in missiles forguidance by radio means.

An object of the present invention is to provide an antenna array whichmay be constructed in a simple and cheap form but which has asubstantial directivity with a good main lobe to side-lobe ratio.

According to the present invention there is provided a microwave antennaarray comprising a conductor adapted to be capable of transmission orreception of en gy. Said conductor being constructed to undulate aboutan axis so as to form a series of transmitting or receiving elementsdisposed on alternate sides of said axis, the amplitude and distancefrom axis point to axis point of said elements being progressivelyaltered along said antenna array, wherein a. the amplitudes of saidelements increase, and their axis point to axis point distancesdecrease, from each end towards the center of said antenna array, I

b. each adjacent pair of elements of said antenna array conformssubstantially to an adjacent pair of elements of a respectiveDolph-Tchebycheff array of a length equal to the length of said antennaarray,

c. each such Dolph-Tchebycheff array has a number of elements related tothe axis point to axis point distance of one of the respective pair ofelements of said antenna array, and

d. each such Dolph-Tchebycheff array has substantially the same mainlobe direction, and substantially the same main lobe to side-lobe ratio.

In order that the invention may be fully understood and readily carriedinto effect it will now be described with reference to the accompanyingdrawings of which:

FIG. 1 is a diagram of one form of antenna array according to thepresent invention,

FIG. 2 is a graph to be used in explaining the design of the array shownin FIG. 1, and

FIG. 3 is another graph to be used in explaining the design of the arrayshown in FIG. 1.

The example of a microwave antenna array according to the presentinvention to be described is an antenna array capable of transmission orreception at microwave frequencies, whose overall directivity pattern issubstantially that of a Dolph- Tchebychetf array, and in which thetransmitting or receiving elements are formed by about an axis conductorof a strip transmission line, constructed so that its undulations aboutan axis comprise excursions which are circularly arcuate and blendedinto one another so as to form a series of circularly arcuatetransmitting or receiving elements disposed on alternate sides of theaxis. In this way such an antenna array can be easily and cheaplyconstructed from a strip of low-loss copper clad laminate, one copperlayer of which forms the ground plane of the transmission line, thelow-loss material forms the dielectric and the other conductor can beproduced by standard photoetching techniques applied to the other copperlayer.

Conventional Dolph-Tchebycheff arrays are well known and have certaincriteria which must be met, namely that the elements of the array areequally spaced and fed with excitation currents of different amplitudewith a linear phase taper along the array; and in Dolph-Tchebychefftheory it is shown, granted these criteria, how to .determine theelement excitation currents so that the side-lobes of the array are ofequal magnitude specified in relation to the main lobe. The main lobebeam width is then given, and is shown to be a minimum for the selectedside-lobe level.

Thus in the example of the present invention to be described, the radiiof the circularly arcuate excursions of the copper strip areprogressively altered along the array so as to cause differentcircularly arcuate elements to radiate at different degrees, and toensure a linear phase taper along the array, the length of thecircularly arcuate elements must also be progressively altered. Atypical example is shown in FIG. 1

which shows a conductor 1 which undulates about an axis 4 so as toprovide a series of circularly arcuate elements disposed on alternatesides of axis 4. It will be seen that their pitches progressively alterover the length of the array in the direction of arrow 5 as their radiiare altered. Conductor 1 is mounted on a slab of low loss material 2,the underside of which is clad with a copper layer 3 which forms theground plane of the transmission line made up of conductor 1 as theother conductor and slab 2 as the dielectric. However it will be seenfrom FIG; 1 that the pitch, or distance between axis crossing points bystrip 1 progressively alters, with the effect that the criterion ofequally spaced elements in a conventional Dolph- Tchebycheff array is nolonger met. The design procedure which ensures that the overalldirective pattern still substantially conforms to that of aDolph-Tchebycheff array will now be described.

The angle 6 of the main lobe to thenor mal to the plane of the array'foran array of circularly arcuate elements of a given radius of curvatureand a given pitch is given by:

where E is the transmission line effective dielectric constant, d thepitch or distance between axis crossing points, r the 'radius ofcurvature, n the spatial harmonic and M) the free space wavelength. Fora desired angle 0 a curve of individual circularly arcuate elementconductance g versus d is plotted, by means of test sections each madeup of constant arcs, with different values of d, ensuring at all timesthat the parameters of the test sections satisfy equation (1) above.From this a curve of g/d versus :1 is plotted, of which a typicalexample is shown in FIG. 2 for which 0=30, E =2.2, and n -2 at afrequency of 10.5 GHz. For a desired beam width of the main lobe, andmain lobe to side-lobe ratio, the length of a suitable conventionalQrrayis defined on the basis of a perfect Dolph- Tchebycheff distribution.For a suitable fractional power load a set of suitable conductancedistributions is computed and plotted as shown in FIG. 3, which shows agraph of element conductance as a function of distance along the arrayfor different numbers of elements. It must be appreciated that eachelement conductance is compensated for radiation and dielectric loss. Itwill be seen from FIG. 3 that all distributions commence at asubstantially identical first element conductance regardless of thenumber of elements in the array. Thus a first conductance (at arraylength =0), which determines the pitch d of the first arc of the arrayto be designed, is then converted to the corresponding value of g/d,which using a graph such as is shown in HO. 2 determines a value of d,which specifies the position of the next axis crossing point in thearray to be designed. Dividing this distance d into the array lengthyields the number of circularly arcuate elements in the array to whichthis part of the array conforms, and thus the next element conductanceis defined from a graph such as is shown by FIG. 3, knowing the numberof elements and the distance d, another number of elements in the arrayand hence yet a next element conductance. This procedure is repeatedalong the length of array, that is to say using each element conductancein turn to determine a new value of d and hence the next elementconductance, and in this way'the synthesized array takes on thecharacteristics of a Dolph-Tchebycheff array. This is also shown in thegraph of FIG. 3, which shows how an experimental array whose circularlyarcuate element conductances along the array, shown by the shapes causethe array to start off at the input-end as if it were aDolph-Tchebycheff array of 35 elements passing to such an array of 41elements at two-thirds along itslength, and then back to a 35 elementarray. Each part of the array, and therefore the array itself, generatesa main lobe at the same angle 6, and the resultant ratio of main lobelevel to side-lobe level is substantially that of the chosenDolph-Tchebycheff array.

The antenna array to which the graph of FIG. 3 conforms, has 31circularly arcuate elements varying from d =().55 inches to d 0.44inches; has an'overall length of inches; main lobe to side-lobe ratio of30 db; an angle of b30; and operates at l0 50GHz. Only a few of thecircularly arcuate elements are shown in FIG. I for convenience ofillustration, but it will be seen how their amplitude reaches a maximumand distance between axis crossing points at their minimum towards thecenter of the array.

It will be appreciated that the invention is not limited to undulationscomprising circular arcs, indeed any undulatory form may be adopted, forexample parts of sinusoids; and nor is it limited to the form ofconstruction described, for example the ground plane may be dispensedwith; or a further ground plane may be fitted, above the undulatingconductor, but with a slit cut in it, permitting transmission orreception over the length of the slit,

What we claim is:

l. A microwave antenna array comprising a conductor adapted to becapable of transmission or reception of energy, said conductor beingconstructed to undulate about an axis so as to form a series oftransmitting or receiving elements disposed on alternate sides of saidaxis, the amplitude and distance from axis point to axis point of saidelements being progressively altered along said antenna array, whereina. the amplitudes of said elements increase, and their axis point toaxis point distances decrease, from each end towards the center of saidantenna array,

h. each adjacent pair of elements of said antenna array con formssubstantially to an adjacent pair of elements of a respectiveDolph-Tchebycheff array of a length equal to the length ofsaid antennaarray, I

c. each such Dolph-Tchebycheff array has a number of elements related tothe axis point to axis point distance of one of the respective pair ofelements of said antenna array, and

d. each such Dolph-Tchebycheff array has substantially the same mainlobe direction, and substantially the same main lobe to side-lobe ratio.

2. An array according to claim 1 in which said transmitting or receivingelements disposed on alternate sides of said axis comprise circularlyarcuate elements.

3. An array according to claim 1 in which said transmitting or receivingelements disposed on alternate sides of said axis comprise partsinusoidal elements.

1. A microwave antenna array comprising a conductor adapted to becapable of transmission or reception of energy, said conductor beingconstructed to undulate about an axis so as to form a series oftransmitting or receiving elements disposed on alternate sides of saidaxis, the amplitude and distance from axis point to axis point of saidelements being progressively altered along said antenna array, whereina. the amplitudes of said elements increase, and their axis point toaxis point distances decrease, from each end towards the center of saidantenna array, b. each adjacent pair of elements of said antenna arrayconforms substantially to an adjacent pair of elements of a respectiveDolph-Tchebycheff array of a length equal to the length of said antennaarray, c. each such Dolph-Tchebycheff array has a number of elementsrelated to the axis point to axis point distance of one of therespective pair of elements of said antenna array, and d. each suchDolph-Tchebycheff array has substantially the same main lobe direction,and substantially the same main lobe to side-lobe ratio.
 2. An arrayaccording to claim 1 in which said transmitting or receiving elementsdisposed on alternate sides of said axis comprise circUlarly arcuateelements.
 3. An array according to claim 1 in which said transmitting orreceiving elements disposed on alternate sides of said axis comprisepart sinusoidal elements.